In recent years, MPEG Surround (ISO/IEC 23003-1:2007) standardized by international organization for standardization/international electrotechnical commission (ISO/IEC) has been adopted in multimedia broadcasting in Japan and digital television broadcasting service in foreign countries. The MPEG Surround involves forming main signal code by encoding an audio signal as an original signal, and forming residual code by encoding a residual signal which is indicative of error components produced by encoding of the original signal. FIG. 19 illustrates an example of a configuration of conventional technology for decoding input data encoded by a method as described above (i.e. coded data obtained by multiplexing the main signal code and the residual code together). In a decoding device 1000 illustrated in FIG. 19, input data is inputted to a data separator 1002 and separated into main signal code and residual code, which are then inputted to a main signal decoder 1004 and a residual decoder 1006, respectively. The main signal decoder 1004 decodes the main signal code to output a main signal. For example, when the main signal code is encoded by use of advanced audio coding (MC) or high-efficiency advanced audio coding (HE-AAC), an MC decoder or HE-AAC decoder for the used coding method is used as the main signal decoder 1004. Meanwhile, the residual decoder 1006 decodes the residual code to output a residual signal. For example, when the residual code is encoded by use of the MC or the like, an MC decoder for this coding method is used as the residual decoder 1006. The decoded main signal and residual signal are inputted to an adder 1008, which then adds these signals together to produce final output data. Such technologies are disclosed for example in Japanese Laid-open Patent Publication Nos. 8-248897 and 2007-72264, International Publication Pamphlet No. WO 2008/066071, and Japanese Patent No. 3871347.
Further, FIG. 20 illustrates an example of a configuration using the HE-MC as an encoding method for a main signal. In a decoding device 1001 illustrated in FIG. 20, input data is inputted to the data separator 1002 and separated into main signal code, auxiliary information code, and residual code, which are then inputted to a lowband main signal decoder 1010, an auxiliary information decoder 1012, and the residual decoder 1006, respectively. Here, the main signal code is a signal obtained by encoding low-frequency components of an original signal. The lowband main signal decoder 1010 decodes the main signal code to output a lowband main signal as lowband components of a main signal. Also, the auxiliary information decoder 1012 decodes the auxiliary information code to output auxiliary information.
In addition, through spectral band replication (SBR) technology, a highband main signal generator 1014 outputs a highband main signal formed of highband components of the main signal by using the auxiliary information and the lowband main signal. Description will now be given with regard to generation of the highband main signal in the highband main signal generator 1014. As illustrated in FIG. 21, the generation of the highband main signal is accomplished by selecting and replicating a predetermined frequency band of the lowband main signal, and making fine adjustments to electric power. Information indicating the predetermined frequency band to be selected, and gain for the fine adjustment of the electric power are contained in the auxiliary information. Then, a main signal synthesizer 1016 synthesizes the lowband main signal and the highband main signal to produce a main signal containing components in a full band. Therefore, the use of the SBR technology enables generating the highband main signal from the lowband main signal and the auxiliary information, and thus, obtaining the main signal code by encoding only the low-frequency components of the original signal, so that the encoding is possible even with low bit rate. Meanwhile, the residual decoder 1006 decodes the residual code by AAC-based or other decoding thereby to output a residual signal. Then, the adder 1008 adds the generated full-band main signal and the residual signal to produce final output data.